Introduce a new associateBy() function

Similar to groupBy(), except that it only keeps the latest value
corresponding to a given key.

Prior art: [Kotlin](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.sequences/associate-by.html)

CHANGELOG.md

@@ -10,6 +10,9 @@
   Assignment to `Equality` with a nullable type is still allowed because of
   covariance. The `equals` and `hash` methods continue to accept nullable
   arguments.
+* Add a top-level `associateBy()` function that converts an `Iterable` to a
+  `Map` by grouping its elements using a function, keeping the last element
+  for each computed key.
 
 
 ## 1.15.0

lib/src/functions.dart

@@ -41,6 +41,19 @@ Map<K, V> mergeMaps<K, V>(Map<K, V> map1, Map<K, V> map2,
   return result;
 }
 
+/// Associates the elements in [values] by the value returned by [key].
+///
+/// Returns a map from keys computed by [key] to the last value for which [key]
+/// returns that key.
+Map<T, S> associateBy<S, T>(Iterable<S> values, T Function(S) key) {
+  var map = <T, S>{};
+  for (var element in values) {
+    map[key(element)] = element;
+  }
+
+  return map;
+}
+
 /// Groups the elements in [values] by the value returned by [key].
 ///
 /// Returns a map from keys computed by [key] to a list of all values for which

test/functions_test.dart

@@ -79,6 +79,23 @@ void main() {
     });
   });
 
+  group('associateBy()', () {
+    test('returns an empty map for an empty iterable', () {
+      expect(associateBy([], expectAsync1((dynamic _) {}, count: 0)), isEmpty);
+    });
+
+    test("keeps the latest element for the function's return value", () {
+      expect(
+          associateBy(['foo', 'bar', 'baz', 'bop', 'qux'],
+              (dynamic string) => string[1]),
+          equals({
+            'o': 'bop',
+            'a': 'baz',
+            'u': 'qux',
+          }));
+    });
+  });
+
   group('groupBy()', () {
     test('returns an empty map for an empty iterable', () {
       expect(groupBy([], expectAsync1((dynamic _) {}, count: 0)), isEmpty);